The objective of this work is the characterization of antibody genes by their identification and detailed mapping. This effort is proceeding along several paths. Heavy chain genetics is being pursued by identifying additional crossover events in the heavy chain gene cluster for the purpose of learning gene order and organization and by identifying and mapping new heavy chain structural genes and other nearby genes. Light chain genetics is proceeding by mapping studies with known light chain genes and efforts to identify new genes by means of structural and serological studies with myeloma proteins. The chromosomal loci of the heavy and light chain gene clusters is being sought by mapping studies with Recombinant Inbred strains and by somatic cell genetic approaches using interspecific hybrids and hybridomas. The NZB x C58 Recombinant Inbred (NX8RI) strains have been inbred for twenty generations and their analysis is underway.